can't speak for cancun, but at dreams pv, we rented table runners for $7 each. used their napkins, plates, etc. brought cake topper and all decorations for table (just little tea lights and holders). we brought fake petals and never even asked - just gave them to the wc to put on the aisle runner. if your resort doesn't let you use petals, try ecofetti - biodegradable confetti. all of their rentals are expensive!!!! i never got a single thing back, so just make sure that you get it back if it's something important to you.
we had 2 cakes, but w/ 30 ppl, it was totally not worth it.